This invention relates to sheet metal stampings formed by progressive stamping tools and, more particularly, to a method and apparatus for the leveling of stamped sheet metal to remove or avoid unwanted distortions.
Sheet metal is a common material used in mass-production manufacturing. Progressive tooling is often used to mass-produce items from a coil of sheet metal by passing the sheet metal through a tool or series of tools, e.g., a stamping press or stretch-forming press, that progressively shape and form the item being produced. Precise control of the feeding distance (or pitch) of the tool that performs the stamping, the feeding rate of the coil of sheet metal, and the frequency (open and shut frequency of the press determined by crankshaft RPM) is required.
In instances where the finished product is punched out of the coil and collected in a bin, such as in the case of circular or semi-spherical metal shells, the remaining portion of the coil of sheet metal is recycled as scrap. In these instances, pilot holes may be punched into the coil in areas of the coil adjacent to the areas being worked by the tooling. The pilot holes may be used to guide and regulate the feeding of the coil through the progressive tooling. In other instances, for example, in the manufacture of bipolar plates for electrochemical fuel cells, the finished product is the stamped coil itself. These coils are typically fed through a stretch-forming press by rollers.
Stretch-forming is a sheet metal forming process that is well known and that has been applied to numerous sheet metal products, for example, to the production of bipolar plates for fuel cells as described in commonly owned U.S. patent application Ser. No. 09/714,526, entitled Fuel Cell Bipolar Separator Plate and Current Collector Assembly and Method of Manufacture, filed on Nov. 16, 2000, the entire disclosure of which is incorporated herein by reference for all purposes.
Stretch-forming is performed in a manner that prevents the drawing-in of adjacent sheet metal into the tooling as the stretch-forming is performed. In the area where the sheet metal is stretched to its desired form, it is elongated well beyond the yield point of the material. Upon opening of the stretch-forming tool, the sheet metal will undergo spring-back or snap-back to relieve residual stress in the sheet metal. The amount of snap-back may be as much as several thousandths of an inch per inch, depending on the mechanical properties of the sheet metal.
In certain cases, peripheral areas of the sheet metal are not stretch-formed by the tooling. For example, when producing continuous components, such as bipolar plates for fuel cells, peripheral edge portions of the sheet metal coil are not stretch-formed and are subsequently processed to operate as seal areas. As the sheet metal coil is progressively stretch-formed as it passes through a stretch-forming press, the snap-back of sheet metal will accumulate as the coil progresses through the press and, therefore, will distort the coil. Effectively, the center area of the coil that is stretch-formed becomes shorter than the adjacent edge portions of the coil that are not stretch-formed. This accumulated distortion creates problems when feeding the coil with coil feeding equipment such as roll feeds, which are used when the use of pilot holes is an impractical method of guiding and regulating the feeding of the coil. For example, pilot holes may be impractical when the material is too thin, or the end product otherwise results in an inability to punch pilot holes in the coil of material.
A need exists for a method and apparatus that will avoid distortion of sheet metal coils that are processed by stretch-forming tooling in a progressive mode, and which use roll feed equipment to advance the coil.
It is an object of the present invention to provide a method and apparatus that reduces or wholly overcomes some or all of the difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.
In the present invention a means is provided to counter the effect of the snap-back of sheet metal that occurs as a stretch-form tool opens.
In accordance with a first aspect, a method of reducing distortion in a stamped sheet metal strip includes the steps of providing a stretch-forming press having a main forming station and a leveling station, the leveling station having a pair of jaws, each jaw being slidably received in a recess inclined at an acute angle with respective to a direction of travel of a strip of sheet metal through the stretch-forming press; stamping a desired pattern on the strip of sheet metal at the main forming station by closing the stretch-forming press; advancing the strip of sheet metal through the stretch-forming press in a direction of travel a desired distance such that the desired pattern is aligned with the leveling station; and closing the stretch-forming press such that the jaws of the leveling station engage the strip of sheet metal and stretch a portion of the strip of sheet metal containing the desired pattern in the direction of travel a selected distance as the jaws slide into the respective recesses when the stretch-forming press is closed.
In accordance with a second aspect, a stretch-forming press for continuous feed sheet metal includes a ram, a base member, and a feed mechanism configured to advance a strip of sheet metal through the stretch-forming press. A forming station has a die configured to form a desired pattern in a strip of sheet metal. A leveling station has a pair of opposed jaws that are slidably received in corresponding recesses of the stretch-forming press. The jaws are oriented at an angle with respect to a direction of travel for a strip of sheet metal through the leveling station.
In accordance with yet another aspect, a stretch-forming press for continuous feed sheet metal includes a ram, a base member, and a feed mechanism configured to advance a strip of sheet metal through the stretch-forming press. A pre-forming station has a pair of spaced apart dies configured to mate with recesses formed in the base member to form alignment recesses in a strip of sheet metal shaped in the stretch-forming press. Each die is surrounded by a jaw, with each jaw biased toward the base member by a biasing member. A main forming station has a pair of spaced apart jaws configured to mate with alignment recesses formed in a strip of sheet metal at the pre-forming station. A die is configured to form a desired pattern in a strip of sheet metal passing through the stretch-forming press. A leveling station has a pair of opposed jaws slidably received in corresponding recesses of the stretch-forming press, and the jaws are oriented at an angle with respect to a direction of travel for a strip of sheet metal passing through the stretch-forming press.
Substantial advantage is achieved through the present invention since distortion of the sheet metal is minimized. These and additional features and advantages of the invention disclosed here will be further understood from the following detailed disclosure of certain preferred embodiments.